v5 for direct mode, with automatic upgrade

This includes storing the current state of the HEAD ref, which git annex
sync is going to need, but does not make sync use it.

Upgrade.hs

@@ -16,9 +16,21 @@ import qualified Upgrade.V0
 import qualified Upgrade.V1
 #endif
 import qualified Upgrade.V2
+import qualified Upgrade.V4
 
-upgrade :: Annex Bool
-upgrade = go =<< getVersion
+checkUpgrade :: Version -> Annex ()
+checkUpgrade v
+	| v `elem` supportedVersions = noop
+	| v `elem` autoUpgradeableVersions = unlessM (upgrade True) $
+		err "Automatic upgrade failed!"
+	| v `elem` upgradableVersions = err "Upgrade this repository: git-annex upgrade"
+	| otherwise = err "Upgrade git-annex."
+  where
+	err msg = error $ "Repository version " ++ v ++
+		" is not supported. " ++ msg
+
+upgrade :: Bool -> Annex Bool
+upgrade automatic = go =<< getVersion
   where
 #ifndef mingw32_HOST_OS
 	go (Just "0") = Upgrade.V0.upgrade
@@ -28,4 +40,5 @@ upgrade = go =<< getVersion
 	go (Just "1") = error "upgrade from v1 on Windows not supported"
 #endif
 	go (Just "2") = Upgrade.V2.upgrade
+	go (Just "4") = Upgrade.V4.upgrade automatic
 	go _ = return True